1. Field of the Invention
The present invention relates to an exhaust gate valve apparatus for a personal watercraft having a hollow pump chamber disposed in a rear portion of a hull. The exhaust gate valve resides at an exhaust port of an exhaust pipe arranged to face the pump chamber.
2. Background Art
One known example of a small watercraft is a personal watercraft of a type driven by water jet propulsion. In a water jet propulsion-driven watercraft, a water jet pump mounted at a rear portion of a hull draws in water from a hull bottom, and expels the drawn-in water rearwardly of the hull to propel the watercraft. In this type of watercraft, it is known to include a structure in which an exhaust port of an exhaust pipe, extended from an engine, is arranged to face a pump chamber, and in which the exhaust port can be opened or closed with a valve body. This known structure is disclosed, for example, in Japanese Laid-open Patent publication No. Hei 7-246996.
The art disclosed in Japanese Laid-open Patent publication No. Hei 7-246996 will be described below with reference to FIG. 10 of the present drawings, in which the basic conventional construction is illustrated.
In FIG. 10, a portion of water jet propulsion-driven watercraft 200 is shown which includes a hollow pump chamber 201 disposed at a rear portion of a watercraft hull. The pump chamber 201 includes a water jet pump, to which an engine is connected. An exhaust port 203 of an exhaust pipe 202, extended from the engine, is made to face the pump chamber 201. A valve body 205, made of a rubber plate, is disposed at a position near the exhaust port 203 via a fixing screw 204.
In the conventional structure, a pressure of exhaust gas (that is, an exhaust gas pressure) is not applied to valve body 205 when, for example, the engine is stationary. Accordingly, the valve body 205 is disposed at a closed position indicated by a solid line in FIG. 10. In this condition, the valve body 205 abuts and confronts an abutment surface 206, maintaining the exhaust port 203 in a closed position.
When the engine is run at high speeds, on the other hand, the exhaust gas pressure builds up, causing the built-up exhaust gas pressure to be applied to the valve body 205. As a result, the valve body 205 is elastically deformed, to take an open position indicated by broken lines in FIG. 10. Opening the exhaust port 203 allows the exhaust gas to be discharged therefrom, and continued pressure from exhaust gases tend keep the exhaust port 203 in an open position as long as the engine continues high-speed operation.
Because the conventional arrangement employs the elastic force of the rubber valve body 205 to close the exhaust port 203 when the engine is stationary, however, it is difficult to ensure a positive contact of the valve body 205 with the abutment surface 206. Unless the positive contact of the valve body 205 with the abutment surface 206 is achieved, a gap may be produced between the abutment surface 206 and the valve body 205. Accordingly, the valve body 205 may be unable to positively and consistently close the exhaust port 203.
As noted above, when the engine is operating at relatively high speed, exhaust gas pressure is applied to the valve body 205. When equilibrium is achieved between the exhaust gas pressure and the elastic force of the valve body 205, the valve body 205 is placed in a stationary state. It is difficult, however, for the exhaust gas pressure to counteract the elastic force of the valve body 205 and thereby widely open the valve body 205. A valve body 205 that is open only insufficiently could hamper proper discharge of the exhaust gas therethrough.
It is, therefore, an object of the present invention to provide an improved exhaust gate valve apparatus for a water jet propulsion-driven watercraft in which the exhaust gate valve apparatus is capable of positively closing an exhaust port with a valve body when an engine is idling and the watercraft is stationary, and in which the exhaust gate valve apparatus is also capable of widely opening the valve body when the engine operates at high speed.